Cellular technologies of today aimed to provide mobile broadband, such as LTE and WiMAX, employ dynamic scheduling of radio resources in frequency as well as in time, in order to achieve efficient utilization of the available resources. With a certain periodicity, typically in the order of 1 ms, a scheduling occasion occurs in a sending radio entity, in which a packet scheduler makes a decision to allocate resources, e.g. in order to transmit a packet to a user terminal. Since a transmission in one cell typically generates interference in a neighboring cell, referred to as inter-cell interference, the dynamic scheduling generates a dynamic interference situation in the network, resulting in radio link quality variations in radio communication.
To compensate for the varying quality of the radio interface, the scheduler employs link adaptation (LA). If the quality is low, link adaptation will adjust the transmission rate, typically by selecting a more robust modulation and/or coding scheme, in order to ensure that the receiver is able to decode the transmission.
How well the link adaptation works depends on how well the scheduler can predict what the quality of the link will be during the transmission. If the link quality is overestimated, the receiver may not be able to decode the transmission and retransmissions become necessary, resulting in increased delay and lower throughput as a consequence. On the other hand, if the link quality is underestimated, the scheduler will select a too robust modulation and coding scheme and transmission will be made at a lower rate than needed.
Since the inter-cell interference has a substantial negative effect on channel quality, it is beneficial if the scheduler can predict the interference generated by neighbouring cells. Furthermore, the interference in many scenarios, i.e. so-called “interference limited” scenarios, puts a limit on how far from the base station antenna transmissions can be made. In order to manage these interference limited scenarios properly, Inter Cell Interference Coordination (ICIC) schemes are often employed. In such schemes, the scheduler, e.g. in a base station in a cell, usually notifies network nodes in the neighbouring cells of in which parts of the frequency band they can expect heavy interference, so that the network nodes in the neighbouring cells can avoid allocating those parts of the spectrum to a user terminal that would otherwise suffer from the interference.
The nature of packet data transmissions generates a very unpredictable interference situation at low to medium traffic loads. Packets often arrive in bursts and are scheduled for transmission in a cell using all available radio resources of the cell during a short time. The radio interface then remains unused until another burst arrives. This creates a situation where user terminals (UEs) in neighbouring cells experience full interference on the whole frequency band during a short period of time, and then experiences no interference for a subsequent period of time. The performance, expressed as bit-rate experienced by the user terminals, in a system having such fluctuating inter-cell interference, is basically the same at 50% load as at 100% load, mainly due to the difficulty in predicting the interference.
The unpredictable interference also makes ICIC difficult. ICIC is most efficient when the frequency resources in a cell are not fully used. The on-off interference situation described above leaves very little room for coordination. Furthermore, the communication between cells is generally not fast enough to allow for coordination of each scheduling decision individually, so the scheduler must often rely on averaging schemes. One solution to this problem is, e.g., a reuse scheme, where the frequency spectrum, e.g. in cell border areas, is divided between neighbouring cells. Such a division could be of a relatively dynamic or of a more static character. However, such a scheme is generally not a very good solution, since the gain in channel quality rarely compensates for the loss in bandwidth due to the use of such a scheme.
There are examples of attempts of addressing the problem of fluctuating inter-cell interference by varying the transmission power, and/or the coding and modulation schemes in order to achieve a non-varying interference during one, or a fraction of one, transmission time interval (TTI). One such example is presented in the patent document EP 1296463 A1. However, such solutions are not applicable in e.g. LTE or WiMAX, where the generated interference, per default, is non-varying during one TTI. Further, it is not possible to vary the transmission power in radio access techniques such as LTE and WiMAX, in a similar way as in WCDMA or GSM.
Consequently, it is a problem that the above-described unpredictable inter-cell interference has a negative effect on throughput, i.e. effective data rate of the communication, radio resource utilisation and/or communication quality.